A fluorescent light tube includes, among other things, and insofar as pertinent to the present invention, a generally cylindrically shaped glass envelope and end caps provided at either end of the glass envelope. Electrical connecting pins are provided on the end caps to connect the light tube to an electrical power source.
As is known to those skilled in the fluorescent light tube art, a light tube is subject to breakage if dropped or released from any appreciable height or if the light tube is struck by another object. Upon breakage, the glass envelope shatters into numerous glass shards, posing a threat of injury to bystanders or anyone attempting to handle the broken light tube. Thus, there has existed a need to apply a coating to fluorescent light tubes which upon the glass envelope being shattered will maintain the end caps in association with the light tube and contain the glass shards between the end caps.
Providing a protective assembly or coating over the exterior of fluorescent light tubes for protecting the light tubes from impact and for retaining glass fragments and debris are known, for example in U.S. Pat. No. 5,536,998, which utilizes a pre-formed semi-rigid transparent tube surrounding the glass envelope and held in place by heat shrinkable material heat shrunk to a portion of the end caps and extending over the pre-formed tube. The pre-formed protective tube is of sufficient internal diameter to allow a uniform air space to form between the protective tube and the glass envelope. The disadvantage of this process is the need to select two different yet compatible materials and provide a means for forming the uniform air space between the protective tube and the glass envelope.
U.S. Pat. No. 5,532,549 teaches coating light tubes by attaching adapters to the end caps and, using these adapters, rotating the light tubes on the surface of a bath containing the coating material. To ensure complete coverage, the light tube must maintain contact with the surface of the bath throughout the coating process.
U.S. Pat. No. 4,507,332 teaches coating light tubes by exposing the glass envelop and a portion of the end caps to a fluidized bed of powdered polymeric material and heating the light tube above the melting temperature of the polymeric material to melt and fuse the powder onto the glass envelop and end caps to form the coating on the light tube. Heating the entire light tube, though, risks loosening the adhesive attaching the end caps to the glass envelope, thus compromising the integrity of the light tube.
Other methods of coating glass envelops include dipping the envelop in a lacquer coating material (U.S. Pat. No. 3,959,525), and spraying silicone coatings onto glass envelops (U.S. Pat. No. 3,902,946). Although adaptable to “batch” type processing, i.e., applying a coating onto several light tubes at one time, these processes require each light tube be attached to an individual manipulator or adapter before undergoing the coating process, thus making the processes slow.